It is well known to prepare photosensitive compositions comprising a photosensitive agent and a polymeric binder and to utilize such compositions in the preparation of lithographic printing plates which can be developed with aqueous neutral or alkaline developing solutions. In such compositions, it is common practice to utilize a diazo resin as the photosensitive agent. The lithographic printing plates typically comprise a metal substrate, such as a substrate of anodized aluminum, which is coated with a photosensitive composition comprising the diazo resin, the polymeric binder and other ingredients such as colorants, stabilizers, exposure indicators, surfactants, and the like.
Many different polymers have been proposed for use as binders in the aforesaid photosensitive compositions. Particular advantages have been achieved by the use of acetal polymers as described, for example, in U.S. Pat. Nos. 4,652,604, 4,741,985, and 4,940,646.
In U.S. Pat. No. 4,652,604, the acetal polymer contains acetal groups of three types, namely six-membered cyclic acetals, five-membered cyclic acetals and intermolecular acetals. In U.S. Pat. No. 4,741,985, the acetal polymer is a mono-acetal containing a six-membered cyclic acetal group. In U.S. Pat. No. 4,940,646, the acetal polymer contains vinyl acetal units derived from an aldehyde that contains hydroxyl groups.
Acetal polymers can be advantageously utilized in lithographic printing plates in which the layer containing the acetal polymer and the diazo resin is the sole radiation-sensitive layer. They can also be usefully employed in so-called "dual layer" plates. In this type of lithographic printing plate, a radiation-sensitive layer containing a diazo resin is coated over an anodized aluminum support and a radiation-sensitive layer containing a photocross-linkable polymer is coated over the layer containing the diazo resin. Such dual layer plates are described, for example, in British Patent No. 1,274,017. They are advantageous in that radiation-sensitive layers containing diazo resins adhere much more strongly to most anodized aluminum supports than do radiation-sensitive layers containing photocrosslinkable polymers. Thus, the enhanced performance provided by photocrosslinkable polymers is achieved without sacrificing the excellent adhesive properties of diazo resin compositions.
Copending commonly assigned U.S. patent application Ser. No. 738,088 filed Jul. 30, 1991, "Photosensitive Compositions And Lithographic Printing Plates Containing Acid-Substituted Ternary Acetal Polymers" by John E. Walls and Larry D. LeBoeuf describes an important advance in the art relating to the use of acetal polymers in radiation-sensitive compositions and lithographic printing plates. The novel acetal polymers described in this patent application are acid-substituted ternary acetal polymers exhibiting improved properties in comparison with previously known acetal polymers.
More specifically, the acid-subsituted ternary acetal polymers described in the aforesaid copending patent application Ser. No. 738,088 are polymers with recurring units represented by the formula: ##STR1## and x=0-8
m=0-8 PA1 y=0-8 PA1 p=0-8 ##STR2## and n.sub.1 =0-25 mole %, preferably 3 to 15 mole % PA1 n.sub.2 =2-25 mole %, preferably 5 to 15 mole % PA1 n.sub.3 =10-70 mole %, preferably 15 to 50 mole % PA1 n.sub.4 =10-60 mole %, preferably 12 to 45 mole % PA1 n.sub.5 =10-45 mole %, preferably 15 to 30 mole %
As indicated by the above structural formula, the acid-substituted ternary acetal polymers can be tetramers, in which the recurring unit comprises a vinyl acetate moiety and first, second and third cyclic acetal groups, or pentamers in which the recurring unit comprises a vinyl alcohol moiety, a vinyl acetate moiety and first, second, and third cyclic acetal groups.
All three of the acetal groups are six-membered cyclic acetal groups, one of them is unsubstituted or substituted with an alkyl or hydroxyalkyl group, another is substituted with an aromatic or heterocyclic moiety, and a third is substituted with an acid group, an acid-substituted alkyl group or an acid-substituted aryl group.
The acid-substituted ternary acetal polymers described above provide lithographic printing plates characterized by improved abrasion-resistance, improved resistance to chemical attack, extended press performance and enhanced rollup properties in comparison with the acetal polymers of the prior art. Lithographic printing plates utilizing the acid-substituted ternary acetal polymers described above as polymeric binders also have the important advantage that they can be processed in aqueous alkaline developing solutions containing very low concentrations of organic solvents. This is highly advantageous in view of the high costs and environmental concerns associated with the use of organic solvents. Since the acid-substituted ternary acetal polymers are fully soluble in aqueous alkaline developing solutions, they avoid the problems encountered with binders that cause the coating to break-up in particulate form.
While the acid-substituted ternary acetal polymers described above provide many important advantages in the manufacture of lithographic printing plates, they suffer from the disadvantage that the ability of the printing plate to resist blinding is not as good as is desired. In the lithographic printing plate art, the term "blinding" refers to inability to adequately take up the printing ink and agents which enhance ink receptivity are often used to avoid or decrease blinding.
More specifically, blinding is an erratic phenomenon occurring during the printing process where the image, in part or totally, loses the ability to carry any or all of the ink being applied. Blinding has been one of the most frustrating and nebulous anomalies pressmen have had to cope with in the printing process. Numerous studies and resulting presentations, papers and articles have been devoted to the subject. The transition to aqueous plates, recycled paper, environmentally safer fountain solutions and less expensive inks have exacerbated the issue of blinding. Present aqueous products are inherently less ink receptive than their solvent processable forerunners. The changes in ink, paper and fountain solution coupled with changes in press design aimed at increased running speeds have created a combination of conditions favorable for permitting blinding. Typically, calcium and barium salts are known to support the occurrence of blinding. This stems from components in the ink being extracted and interacting with components in the fountain solution and/or ink to form hydrophilic salts that either chemically or physcially become part of the image.
Certain polyvinyl acetals, for example, commercially available FORMVAR and BUTVAR resins, are less susceptible to blinding than the above-described acid-substituted ternary acetal polymers. However, the problem with these resins is that a developer with a high content of organic solvent must be used. Particulate development and the drawbacks associated therewith are then encountered. The goal is to achieve a system in which the coating is inherently highly oleophilic, develops in a dissolving manner and is capable of use with a developer having a maximum content of water.
It is toward the objective of providing improved photosensitive compositions and improved lithographic printing plates, capable of effectively meeting the needs of the lithographic printing plate art, that the present invention is directed.